The invention relates to the field of induction heating, and more particularly to channel furnaces. Channel furnaces typically include an induction heater spaced by a metallic bushing from a refractory material defining the channel of the channel furnace. The construction and configuration of the metallic bushing in a manner to reduce power losses due to eddy currents is the particular subject of the invention, however the invention can be appreciated for use in other environments where it is desired to minimize power losses attributable to leakage fluxes.
Channel induction furnaces are well known for heating an electrically conductive material, such as a metal like aluminum, where the metal is disposed to form a metal loop around a coil and core assembly in a manner to essentially function as a single turn secondary winding thereof. When power is applied to the coil, a magnetic flux is generated into a laminated iron core and a voltage and current are induced in the metal loop. The molten metal in the loop defined by the channel of the furnace is retained and spaced from the coil by a refractory lining. A bushing is interposed between the core and coil assembly and the refractory to space the refractory from the coil. The bushing is often water cooled to enhance its ability to protect the coil from the heat of the furnace. It is well known that to prevent the bushing from acting as a short-circuited secondary winding, the bushing will include a gap or slit disposed along the entire longitudinal length of the bushing. However, such prior known bushing configurations have suffered from problems of undesirable power losses, not from the main flux of the core, but from the channel current flux and the leakage flux of the coil.
The magnetic fluxes in a conventional channel inductor are as shown in FIG. 2. The main flux 50 in the core does not generate any current in the bushing 40 since it is slit along its length by a gap 42 (shown in FIG. 3), but the leakage field 52 and channel current flux 54 do. For analysis purpose, the leakage field will be discussed as being divided into an axial component and a radial component.
The axial component refers to the longitudinal flux 56 parallel to the bushing. It comprises contributions from both the coil 18 and the channel 22. This component induces a current 58 which circulates within the bushing thickness (see FIG. 3). Hereafter this current will be referred to as the "layer current".
On the other hand, the radial component refers to the transverse flux 60 (FIG. 4) penetrating through the bushing 40. This flux is mainly generated by the channel current I.sub.2. It induces the double-loop current pattern 62 in the bushing plate. Hereafter this current will be referred to as the "plate current".
Without changing the existing bushing configuration, the only parameters that can be adjusted to minimize eddy current power losses are bushing thickness and resistivity.
Considering the base relationship, P=I.sup.2 R, the best way to minimize power loss (P) is to reduce the eddy current (I). If reducing the current is difficult, then the only way left is to reduce the resistance (R).
It is very easy to reduce the layer current 58 of FIG. 3, i.e., reduce the bushing thickness and/or increase the resistivity. When the thickness is smaller than half of a penetration depth, the power loss will be negligible due to cancellation between the opposite currents.
Penetration depth is defined by the equation: ##EQU1##
It is, however, very difficult to minimize the plate current 62 of FIG. 4, since the impedance of such a big loop is inductance dominant. Reducing thickness and/or increasing resistivity has little influence on plate current without going to extremes. For example, it takes a stainless steel bushing of thinner than 0.098 of an inch to reduce the current and the power loss. Such a thin bushing may create mechanical and heat conduction problems. Considering the difficulty in reducing current, the practical method in this case is to reduce the resistance. This means that a good conductor such as copper with a sufficient thickness could be used.
Since the optimization requirements in the above two cases are contradictory, there exists an optimum thickness for a given material. For copper, the optimum thickness of a typical bushing is around 0.39 inch. Further increase in thickness will cause the layer current and hence the total power loss to increase; any further decrease in thickness will cause the resistance of the plate current loop and hence the total power loss to increase.
The above analysis shows that the ability to achieve a reduction in bushing losses is limited mainly because it is so difficult to reduce the plate current. To do so, it is necessary to cut off the current path. This can only be achieved by breaking up the solid bushing.
The subject invention overcomes the problems of prior known bushing configurations to provide a plurality of new bushing configurations, all of which reduce power losses while providing a suitable separation and protection of the coil and core assembly from the refractory lining.